Hopefully these screenshots will help you plus my explanation of how I am understanding it.

The left hand half is your health, the right hand is your targets.

The top triangles are your current health percentage from the outer edge with 0% being in the center.

The bottom triangle is your targets health percentage in relation to yours. I think. Although one of he screenshots gives the impression it may be your power percentage. I think Scott may have to confirm that or someone else that knows for sure.

The bottom arrow is a ratio of player health vs target health. If you have 100% health and your target has 60% health, then the bottom arrow would be halfway between the center of the bar and the left side of the bar. If you are at equal health, it would be right in the middle. If your target has more health than you it would move to the right of center. And lastly, if one of you is dead, it would be all the way to the left or right, depending on who is dead.